This invention relates to battery operated inverters and, in particular, to an inverter for driving an electroluminescent (EL) panel having one or more EL lamps, one or more light emitting diodes (LEDs), and providing a bias voltage for a liquid crystal display.
An EL lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer may include a phosphor powder or there may be a separate layer of phosphor powder adjacent the dielectric layer. The phosphor powder radiates light in the presence of a strong electric field, using very little current. Because an EL lamp is a capacitor, alternating current must be applied to the electrodes to cause the phosphor to glow, otherwise the capacitor charges to the applied voltage, the current through the EL lamp ceases, and the lamp stops producing light.
In portable electronic devices, automotive displays, and other applications where the power source is a low voltage battery, an EL lamp is powered by an inverter that converts direct current into alternating current. In order for an EL lamp to glow sufficiently, a peak-to-peak voltage in excess of about one hundred and twenty volts is necessary. The actual voltage depends on the construction of the lamp and, in particular, the field strength within the phosphor powder. The frequency of the alternating current through an EL lamp affects the life of the lamp, with frequencies between 200 hertz and 1000 hertz being preferred. Ionic migration occurs in the phosphor at frequencies below 200 hertz. Above 1000 hertz, the life of the phosphor is inversely proportional to frequency.
The prior art discloses several types of inverters in which the energy stored in an inductor is supplied to an EL lamp as a small current at high voltage as the inductor is discharged either through the lamp or into a storage capacitor. The voltage on a storage capacitor is pumped up by a series of high frequency pulses from the inverter. The direct current produced by inverter must be converted into an alternating current in order to power an EL lamp. U.S. Pat. No. 4,527,096 (Kindlmann) discloses a switching bridge for this purpose. The bridge acts as a double pole double throw switch to alternate current through the EL lamp at low frequency. U.S. Pat. No. 5,313,141 (Kimball) discloses an inverter that produces AC voltage directly. A plurality of inverters are commercially available using either technology.
Many portable electronic devices, such as personal digital assistants and cellular telephones, use LEDs for illuminating a liquid crystal display (LCD) and an EL lamp for illuminating a keypad. The LEDs are typically powered by a driver, the liquid crystal display has its own driver, and the EL lamps are powered by an inverter. A driver is required for the LEDs because portable electronic devices use a 1.5-3 volt battery and LEDs have a minimum forward voltage drop of 1.8 volts. If LEDs are connected in series to minimize current, then the voltage requirement increases accordingly. If the LED is switched by a transistor, then the forward voltage drop of the transistor further increases the required supply voltage. In addition, LEDs require a ballast or current limiting device, such as a resistor or inductor, which dissipates power. Inductors are more efficient than resistors but are relatively expensive components and it is desired to minimize the number of inductors required by a circuit. The liquid crystal display requires bias voltages.
It would provide a significant cost savings if LEDs, LCDs, and EL lamps could be driven from the same driver. The problem is that EL lamps need 100 volts or more AC, LEDs need from 5-12 volts DC, and LCDs typically need a bias of 5-30 volts.
In view of the foregoing, it is therefore an object of the invention to provide an inverter for providing plural voltages for a personal electronic device.
Another object of the invention is to provide an inverter for driving LEDs, LCDs, and EL lamps by adapting an existing, commercially available inverter.
A further object of the invention is to provide an inverter for driving LEDs, LCDs, and EL lamps by modification to either type of existing inverter for EL lamps.
The foregoing objects are achieved in this invention in which an inverter for producing plural voltages is tapped to provide voltage or current for driving one or more EL lamps, LEDs, and an LCD. One side of a pump inductor is tapped for a voltage that drives one or more EL lamps and the LCD.